System for producing cementitious materials from ferrous blast furnace slags

ABSTRACT

Processes and systems for producing cementitious materials from ferrous blast furnace slags. Processes comprise mixing of a source of CaO with molten slag flowing from at least one slag tap hole of a blast furnace. Sufficient CaO is mixed with the molten slag to raise the ratio of CaO to SiO 2 , the C/S Ratio, of the slag to between about 1.06 and 1.25, while maintaining the Base Number of the molten slag less than about 1.55. The slag is then water granulated and ground to a predetermined degree of fineness. An activator then is added to the ground granulated slag. The C/S Ratio of the granulated slag determines the degree of fineness to which the granulated slag must be ground to achieve a desired hydration rate in the resultant cementitious material. Cementitious materials with C/S Ratios in this range may be ground less finely, yet possess the same hydration rates, as other more finely ground cementitious materials.

This application is a division of application Ser. No. 08/023,378, filedFeb. 26, 1993 now U.S. Pat. No. 5,374,309.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improvements in the cementitious properties ofgranulated, ferrous blast furnace slags used to produce cementitiousmaterials. Specifically, it relates to a process for mixing a source ofcalcium oxide (CaO) with molten ferrous blast furnace slag, such thatthe hydraulic reactivity of the slag after granulating is enhanced. Whensuch slag is ground, cementitious properties that are the same as orsimilar to those of more finely ground cementitious materials can beachieved at a lower specific surface area, thereby saving production andenergy costs. Moreover, improved properties can be achieved by grindingsuch slag to the same or similar specific surface area as that of aground, granulated blast furnace slag produced without the addition of asource of CaO. The invention further relates to a system for performingthis addition.

2. Description of the Related Art

Portland Cement is the most widely used cementitious material. It ismanufactured by heating a finely ground and controlled blend oflimestone (or another source of CaO, such as chalk) with an argillaceousmaterial such as shale or clay to a temperature of about 3180° F. (1500°C.). When necessary, other sources of silica (SiO₂), such as sand;alumina (Al₂ O₃), such as bauxite; and ferric oxide (Fe₂ O₃), such aspyrites, may also be added. After it is cooled, the resultant clinker ismixed with gypsum (or a mixture of anhydrite and gypsum) and ground to afine powder.

There are a number of different processes for making cementitiousmaterials. In a wet process, the mixture of materials is ground withwater to produce a slurry which is then fed to a rotary kiln. In thisprocess, the fuel consumption in the kiln is generally high due to theneed to dry, as well as to heat the materials. In a traditional dryprocess, the materials are ground dry to form a powder which is then fedto a kiln. This process generally has a lower fuel consumption than wetprocesses. Variants of these processes include a semi-wet process, inwhich water is partially removed from the slurry by filtration to form acake of ground materials which then is fed to the kiln, or the Lepolprocess, in which a dry powder is first nodulized with the addition ofsome water after which the nodules are fed to the kiln.

In a suspension preheater process, the materials are ground dry to forma powder which is passed through a series of cyclones before it entersthe kiln. In these cyclones, the powder mixes with waste gases from thekiln and is gradually heated, such that the powder may be reach atemperature of about 1590° F. (750° C.). As a consequence of thispreheating, the fuel consumption in this process may be lower than inthe processes described above.

The suspension preheater process may be modified by the addition of acombustion chamber between the cyclones and the rotary kiln in whichabout 50 percent of the total fuel required to produce a clinker can befired. This process is known as a precalciner process. In this process,the powder may be heated to about 2014° F. (950° C.) before it entersthe kiln. An advantage of this process is that a higher output may beattained for a given kiln size. Even in the more thermally efficientsuspension preheater and precalciner processes, however, the clinkerforming step is usually the most energy intensive in processes formaking cementitious materials.

When the finely ground blend of materials is heated in any of the aboveprocesses, the argillaceous materials usually begin to de-hydrate atabout 1272° F. (600° C.), and the calcareous materials may decarbonateat about 1908° F. (900° C.). At about 2756° F. (1300° C.) the mixtureusually begins to melt, so that by about 3180° F. (1500° C.) about 25percent of the mixture is molten. This liquid encourages the formationof clinker and promotes chemical reactions between the CaO, SiO₂, Al₂ O₃and Fe₂ O₃, to form four chemical compounds commonly referred to asclinker minerals, which are described in Table I. All percentagesexpressed herein are by weight.

                  TABLE I                                                         ______________________________________                                                    Short-             Typical Levels in                                          hand               Ordinary Portland                              Chemical    Nomen-   Mineral   Cement (OPC)                                   Formulae    clature  Name      (%)                                            ______________________________________                                        3CaO.SiO.sub.2                                                                            C.sub.3 S                                                                              Alite     45-70                                          2CaO.SiO.sub.2                                                                            C.sub.2 S                                                                              Belite    6-30                                           3CaO.Al.sub.2 O.sub.3                                                                     C.sub.3 A                                                                              Aluminate 5-12                                           4CaO.Al.sub.2 O.sub.3.Fe.sub.2 O.sub.3                                                    C.sub.4 AF                                                                             Ferrite   5-12                                           ______________________________________                                    

When clinker containing these minerals is ground to a fine powder andmixed with water, the aluminate reacts rapidly causing the mixture toset. For this reason, clinker may be ground with about five (5) percentgypsum (or another appropriate source of calcium sulfate (CaSO₄)) tocontrol this reaction, so that grouts, mortars, or concretes preparedfrom cementitious materials can be placed and compacted (or poured)before hardening commences.

After setting, the cementitious material gains strength as the calciumsilicate compounds hydrate. Alite is the main strength giving mineral inPortland Cement contributing to both early and late strengths, whereasbelite is less reactive and only contributes to strengths typicallyfourteen (14) days after placement (or pouring). Both minerals reactwith water to form a non-crystalline, calcium silicate hydrate orC--S--H gel. In this gel, the ratio of CaO to SiO₂ (hereinafter the "C/SRatio") is less than that in the unhydrated clinker minerals, and theCaO liberated as the minerals hydrate combines with water to formcalcium hydroxide (CA(OH)₂).

Cementitious material users are particularly interested in setting andstrength development characteristics, and for this reason, minimumstrengths and both minimum and maximum setting times are specified inthe ASTM (American Society For Testing and Materials) C150 standardspecification for Portland Cements. In addition to the clinker mineralsdescribed in Table I, there are a number of minor components which formin the clinker from impurities in the raw materials or fuel. These minorcomponents can influence both the clinker forming process and thehydraulic reactivity and cementitious properties of the resultantcementitious material. The level of alkalis, notably K₂ O and Na₂ O,present in Portland Cement may be of particular concern. If cementitiousmaterials are combined with aggregates containing reactive SiO₂ to makeconcrete, the alkalis from the cementitious material may react with thisSiO₂ to form an expansive alkali silica gel which can lead to thecracking and break up of the concrete structure. Detecting and avoidingthe use of aggregates containing reactive SiO₂ is difficult. Therefore,in order to avoid this problem, cementitious materials with a specifiedlow alkali content are commonly used. Thus, a maximum equivalent Na₂ Oof about 0.60 percent is included as an optional limit in the ASTM C150.

One way of safely using cementitious materials containing more thanabout 0.60 percent equivalent Na₂ O with aggregates containing reactiveSiO₂, and of avoiding excessive expansion while at the same timereducing the total energy consumption to manufacture the cementitiousmaterials, is to mix the Portland Cement with other materials which arenot cementitious by themselves, but which are capable of reacting withthe alkalis and Ca(OH)₂ liberated, as the Portland Cement hydrates toform additional cementitious hydrates. Suitable materials for thispurpose include pozzolanic materials, such as power station fly ash andcertain types of naturally occurring volcanic material, or latentlyhydraulic materials, such as ground, granulated blast furnace slag.

Slag, as used herein, refers to the material that remains after thesmelting of a metallic ore, a process which entails the reduction of theore to a molten state. Smelting of iron ore generally involves thecombination of iron ore; a source of carbon, generally coke; and a flux,such as limestone, in a blast furnace. The terms "ferrous slag" or"ferrous blast furnace slag" refer to the slag that remains after thesmelting of iron ore.

Pozzolanic materials are generally low in CaO and contain SiO₂ and Al₂O₃ in an active--usually vitreous--form, which react with the Ca(OH)₂released by the hydrating Portland Cement. Latently hydraulic materialscontain sufficient CaO, SiO₂, and Al₂ O₃ to form their own calciumsilicate and aluminate hydrates. The reactions may only be activated insuitably alkaline conditions, such as are achieved in the presence ofhydrating Portland Cement or by adding small amounts of alkalis, such assodium hydroxide or sodium carbonate, to the latently hydraulicmaterial.

Consequently, pozzolana and latently hydraulic materials, even whenmixed with Portland Cement, do not react as fast as Portland Cement andas a result, contribute more to the late strength than to the earlystrength of a cementitious material. An advantage of their lower earlyreactivity, however, is that pozzolana and latently hydraulic materialsproduce less heat than hydrating Portland Cement during the earlyreaction stages. If only Portland Cement were to be used, the heatproduced in large structures could subject the structure to excessivethermal stresses during the initial hardening and result in crackformation.

In addition to the advantages outlined above, mixing Portland Cementwith ground, granulated blast furnace slag can improve the workabilityof concrete, mortar, or grout, thereby making it easier to place andcompact (or pour). Mixing can also reduce the permeability of hardenedconcrete, mortar, or grout, thereby improving its durability. At highpercentage slag mixtures, a resultant concrete, mortar, or grout mayhave sulfate resisting properties equal to or better than an ASTM Type Vcement, as defined in ASTM C150.

Granulated ferrous blast furnace slag may be mixed with Portland Cementin various ways. The slag may be interground with the Portland Cementclinker and gypsum to produce a cementitious material. This is a commonpractice in continental Europe. Alternatively, it may be ground aloneand then mixed with Portland Cement as is common practice in the UnitedStates of America and the United Kingdom. An advantage of having theground, granulated blast furnace slag available separately is that itcan be activated by the addition of alkalis, such as sodium hydroxide orsodium carbonate, without any Portland cement, for certain specificapplications such as soil stabilization, as described in U.S. Pat. No.4,761,183, and the solidification of mud for well cementing operations,as described by Cowan et al. in Paper SPE 24575 presented at the 67thAnnual Technical Conference and Exhibition of the Society of PetroleumEngineers held in Washington, D.C. in October 1992.

In order to acquire hydraulic properties, blast furnace slag must bequenched rapidly to preserve the molten slag in a vitreous state. Twoprocesses that are commonly used are granulation and pelletization. Ingranulation, the slag is quenched by the injection of a large quantityof water under pressure into the slag, i.e., water granulation. If theslag temperature prior to quenching is above the liquidus, watergranulation yields a wet sand like material with a high degree ofvitrification. The water is injected under pressure and at asufficiently low temperature to quench the slag and break up the moltenslag into small pieces. Because of its increased surface area, themolten slag cools quickly. In a pelletization process, the molten slagis directed onto a spinning drum together with a small quantity ofwater. The slag is then projected into the air and solidifies as small,round, and frequently hollow pellets. As a consequence of the slowerquench rate, the degree of vitrification of pelletized slags is normallylower than that of granulated slags. If permitted to cool slowly orallowed to form large, slow cooling pieces, molten slag may crystalizeand exhibit little or no cementitious qualities.

Despite the advantages gained by mixing Portland Cement with ground,granulated blast furnace slag, strength development remains an importantproperty, and for this reason, it has been traditional in the UnitedStates of America to compare the strength measured in psi after aboutseven (7) and twenty-eight (28) days curing of a 50/50 blend of PortlandCement and ground, granulated blast furnace slag with unblended PortlandCement, as outlined in ASTM C989. This comparison has been expressed asa slag activity index (SAI). This ratio of strengths permits thecomparison of the hydration rates of various types of cementitiousmaterials.

The glass content and the composition of the slag, however, will alsoprovide a guide to its hydraulic reactivity, and it is also common toconsider the C/S Ratio, or the ratio of the combined CaO, MgO, and Al₂O₃ contents to the SiO₂ content (hereinafter the "Hydraulic Index"). Ingeneral, slag with the highest glass content, the highest C/S Ratio, andHydraulic Index is preferred for use with Portland Cement. Nevertheless,the higher the C/S Ratio and Hydraulic Index the higher the slag meltingtemperature and the more difficult it is to quench the molten slag to avitreous mass, without formation of crystalline phrases. Because slag isa by-product from the iron smelting (and steel-making) process, itscomposition is largely dictated by the composition of the iron ore,coke, and other raw materials and the efficiency of furnace operation.

Ferrous slag produced in the United States of America, however, usuallyhas a C/S Ratio of about 1. Nevertheless, as the CaO content and thus,the C/S Ratio of the slag increase, the viscosity of the slag alsoincreases. As the viscosity of the slag increases, however, theefficiency of operation of the blast furnace may decline. Further,sources of CaO may be relatively expensive components in the smelting ofiron. Iron smelters, therefore, may seek to limit the amount of anysource of CaO that they add to the smelting process, so that they canminimize the costs of smelting and maximize the efficiency of theirfurnaces.

For example in the United States of America, iron smelters will commonlymonitor the Base Number of the molten slag produced by their furnaces.

    Base Number=(CaO+MgO)/SiO.sub.2.

When the Base Number exceeds about 1.55 and the C/S Ratio exceeds about1.25, the viscosity of the slag may have significant adverse effects onblast furnace operations and efficiency. In the United States ofAmerica, where competition in the iron and steel industry is intense,iron smelters generally operate their furnaces to produce slag withrelatively low C/S Ratios and Base Numbers. In other countries, however,C/S Ratios or Base Numbers may be higher due to furnace and raw materialdifferences. As a consequence, the hydraulic reactivity of blast furnaceslag may be below its potential and is often variable. To produce asuitable ground, granulated blast furnace slag from such slag, it isnecessary to finely grind the slag to achieve very high surface areas toensure it conforms with ASTM C989 and to vary the fineness of the grind,so that the variability of the product is reduced to a level acceptableto users. This results in high energy costs to grind the slag, loweroutput, and higher testing and manpower costs to monitor the slagquality and to adjust slag grinding operations.

Applicant has developed a process and system for incorporating a sourceof CaO into molten slag, such that the C/S Ratio of the slag rises, butthe slag can still be successfully granulated to produce a highlyvitreous mass. Such slag exhibits improved hydraulic reactivity.Therefore, Applicant's process permits less finely ground, granulatedslags to be used to produce cementitious materials possessing qualitiespreviously available only from more finely ground, granulated slags;thus reducing energy costs. Alternatively, Applicant's process permitsthe production of ground granulated slags with improved properties overprevious granulated slags which were ground to the same fineness. Withsuch slags, it is possible in certain applications to mix more ground,granulated blast furnace slag with the Portland Cement and still producehigh quality cementitious materials.

SUMMARY OF THE INVENTION

This invention relates to a process for producing a cementitiousmaterial from molten ferrous slag. In this process, a source of CaO ismixed with the molten slag at a mixing point, so that a ratio of CaO toSiO₂, the C/S Ratio, in the molten slag in a range of about 1.06 to 1.25is obtained, and the slag has a Base Number less than about 1.55. Themolten slag is then granulated, and the granulated slag is ground to apredetermined degree of fineness. The degree of fineness is determinedby the chemical composition of the granulated slag and the hydraulicqualities desired in the cementitious material. An activator, which maybe an alkali selected from a group consisting of potassium hydroxide,sodium hydroxide, and calcium hydroxide, may then be added to theground, granulated slag to produce the cementitious material.

The source of CaO may be quicklime, such as dolomitic quicklime; lime;or limestone. Quicklime is a calcined material the major portion ofwhich is CaO (or CaO in natural association with a lesser amount ofMgO). Lime generally refers to hydrated or slaked lime which is a drypowder obtained by treating quicklime with enough water to satisfy itschemical affinity for water under the conditions of its hydration. Limealso may be associated with excess water, resulting in a moist orslurried state. Limestone is an uncalcined material the major componentof which is calcium carbonate (CaCO₃).

Additionally, the molten slag's chemical composition can be determined,and the molten slag's rate of flow can be measured before the source ofCaO is mixed with the molten slag.

In addition, the slag can be granulated by injecting a fluid, such aswater, or a combination of fluids, such as water and air, into the slagat a sufficiently low temperature, in sufficient quantity, and undersufficient pressure to quench and break up the molten slag.

If the source of CaO is lime, it may also be necessary to mix a lowvolatile fuel, e.g., coke fines or rice hulls, with the lime and themolten slag, whereby sufficient heat is generated in the molten slag tode-hydrate the lime. It also may be necessary to add oxygen (O₂) to themolten slag to facilitate combustion of this low volatile fuel.Alternatively, if the source of CaO is limestone, it may be necessary tomix a low volatile fuel, e.g., coke fines or rice hulls, with thelimestone and the molten slag, whereby sufficient heat is generated inthe molten slag to decarbonate the limestone. It again may also benecessary to add oxygen (O₂) to the molten slag to facilitate combustionof the low volatile fuel. Further, even when the source of CaO isquicklime, it may be necessary to add a low volatile fuel to the moltenslag to generate heat in the slag to promote fusion.

Further, a volatile fuel, e.g., wood, may be added to the molten slag,whereby the slag is percolated by gases released during the combustionof the volatile fuel. This percolation may accelerate the mixing of themolten slag and the source of CaO and serve to break up any crystallinelayer created by the addition of an excess quantity of fines of thesource of CaO to the molten slag.

This invention also relates to a system for producing a cementitiousmaterial from molten ferrous slag flowing from a source of such slag,e.g., a ferrous blast furnace. It is of course possible to re-meltferrous slag that has previously been generated and allowed to cool.This, however, entails additional energy costs.

The system comprises a slag runner for directing the molten slag to amixing point; an injection bin; means for delivering a source of CaO toa centrally located storage bin; and means for transferring the sourceof CaO from the storage bin to the injection bin, whereby the quantityof fines of the source of CaO that are produced during the transfer isstrictly controlled. If excessive fines are produced and mixed with themolten slag, they will cause the C/S Ratio at the surface of the slag torise rapidly. The surface of the molten slag may then crystalize andprevent mixing of the source of CaO with molten slag beneath thiscrystallized surface.

Examples of means suitable for delivering the source of CaO to thecentrally located storage bin may include a pneumatic pipeline and aspur and railcars fitted with attachments for offloading bulk drymaterial. Suitable railcars may include covered hoppers capable ofcarrying about one hundred tons of a source of CaO.

The system further may comprise means for measuring the molten slag'srate of flow in the slag runner and means for determining the moltenslag's chemical composition. A flowmeter is an example of a mechanismsuitable for measuring the molten slag's rate of flow. The molten slag'srate of flow in the slag runner may also be less accurately, butadequately, measured by determining the depth of the molten slag in theslag runner, such as by depth gradations on the slag runner, and bydetermining the rate of flow of the molten slag past a point on the slagrunner. Means for determining the molten slag's chemical content maycomprise a sampling device, a sample transporting device, and testingequipment capable of identifying and measuring the quantity of chemicalcomponents within a sample of molten slag. The system further maycomprise means for mixing the source of CaO with the molten slag in theslag runner at a predetermined rate, means for granulating the moltenslag, means for measuring the granulated slag's chemical composition,and means for grinding the granulated slag to a predetermined degree offineness. In one embodiment, the means for granulating may comprise agranulating station at which a fluid at a temperature sufficiently lowto quench the slag can be injected into the molten slag in sufficientquantity and at sufficient pressure to granulate the molten slag.

Additionally, the system may comprise means for collecting and removingcaustic dust, e.g., dust containing CaO, released during the mixing ofthe source of CaO with the molten slag. The means for collecting andremoving this caustic dust may comprise a dedusting chamber surroundingthe mixing point; a fan, whereby air and the dust are drawn from thechamber; and a scrubber or dry dust collector, whereby the dust and airdrawn from the chamber are separated.

In addition, the means for transferring the source of CaO from thestorage bin to the injection bin may comprise a pneumatic transfervessel in communication with the storage bin and capable of receiving aquantity of the source of CaO under pressure from the storage bin. Itmay further comprise at least one dry air compressor to pressurize thepneumatic vessel and for propelling the source of CaO through the pipingconnecting the vessel to the injection bin and a junction for providingdry compressed air from the dry air compressor or compressors to thepiping.

As noted above, the source of CaO may be quicklime, such as dolomiticquicklime; lime; or limestone. If the source of CaO is lime, the systemcan further comprise means for mixing a low volatile fuel with the limeand the molten slag, whereby sufficient heat is generated in the slag tode-hydrate the lime. It also can comprise means for injecting oxygen(O₂) into the molten slag, such as by a ceramic oxygen lance coupledwith a source of oxygen (O₂).

If, however, the source of CaO is limestone, the system may furthercomprise means for mixing a low volatile fuel with the limestone and themolten slag, whereby sufficient heat is generated in the slag todecarbonate the limestone. It also may comprise means for injectingoxygen (O₂) into the molten slag, such as by a ceramic oxygen lancecoupled with a source of oxygen (O₂). Further, even when the source ofCaO is quicklime, it may be necessary to add a low volatile fuel oroxygen (O₂), or both, to the molten slag in the manner described above,to generate heat in the molten slag. A suitable mechanism for mixing alow volatile fuel with the molten slag could include a volumetric feederof sufficient capacity located adjacent to the mixing point.Alternatively, the low volatile fuel may be mixed with the source of CaOin a single feeder and mixed simultaneously with the molten slag.

It is an object of this invention to save energy and energy costs in themanufacture of cementitious materials. It is a feature of this inventionthat a source of CaO is mixed with molten slag to achieve fusion. It isan advantage of this invention that depending on the source of CaOchosen little or no fuel need be added in this process (or by thissystem) to achieve the desired fusion nor need the operations of theblast furnace be altered to produce molten slag at a higher temperature.

It is also an object of this invention to reduce energy use, energycosts, work, and expenses associated with the grinding of the granulatedslag. It is a feature of this invention that slag with C/S Ratiosbetween about 1.06 and 1.25 and Base Numbers less than about 1.55 thathave been granulated by the injection of a sufficient quantity offluid(s) at a temperature sufficiently low to quench the molten slag andunder sufficient pressure have increased hydraulic reactivity over othergranulated slags at all degrees of fineness of grind. It is an advantageof this invention that cementitious materials produced by this processand in this system possess the same qualities as other more finelyground cementitious materials. This also results in higher productionrates of cementitious materials by this process.

It is a further object of this invention that cementitious materials areproduced with hydraulic qualities similar to Portland Cement, but at alower cost. It is a feature of this invention that the cementitiousmaterial is manufactured from ferrous blast furnace slag. Further,advantages of this invention are that more slag--a waste product--can berecycled and that the operation of the blast furnace need not bemodified to produce slag suitable for this process.

It is an additional object of this invention to utilize slag producedfrom blast furnaces which produce slag with lower C/S Ratios and lowerBase Numbers, such as those currently operated in the United States ofAmerica, to manufacture cementitious materials exhibiting improvedhydraulic reactivity. It is a feature of this invention that thepredetermined amounts of a source of CaO are mixed with the molten slagafter it leaves the blast furnace to improve the hydraulic qualities ofa resultant cementitious material. It is an advantage of this inventionthat blast furnaces can continue to operate within the parametersimposed by competitive market forces and produce molten slag suitablefor the manufacture of cementitious materials possessing qualitiessimilar to those of Portland Cement.

Other objects, features, and advantages will be apparent when thedetailed description of the invention and the drawings are considered.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart for a description of a preferred embodiment ofthe process of the invention.

FIG. 2 is a graph depicting the relation between median particle sizeand ball mill grinder production rates in a preferred embodiment of theprocess.

FIG. 3 is a schematic view showing a preferred embodiment of the systemof the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the flow chart indicates, a preferred embodiment ofthe process for enhancing the cementitious properties e.g., thehydration rates or hydraulic reactivity, of ferrous blast furnace slags.The process may save energy costs by reducing the grinding and/or thequantity of activator, particularly Portland Cement, required to producecementitious materials possessing predetermined hydraulic qualities.

Molten ferrous slag is a by-product of the smelting of iron, and mostblast furnaces are equipped with one or more slag runners, i.e.,channels made of fire brick, castable refractory material, or similarmaterials, which are built with constant gradients, to direct moltenslag away from the blast furnace tap hole(s). Initially, molten ferrousblast furnace slag (hereinafter "molten slag") may be drawn from anoperating blast furnace. Molten slag flows from a blast furnace at atemperature of between about 2600° and 2650° F. (1226° and 1250° C.).The amount of molten slag produced and its chemical composition maydepend on the quality of the iron ore smelted and the amount of ironproduced by the blast furnace operations. Further, the chemicalcomposition of the slag may also depend on the quality of theingredients used in the smelting process. For example, limestone iscommonly used as a fluxing material. If more limestone is added to thefurnace, the CaO content of the resultant slag may increase.

Molten slag commonly has a C/S Ratio in the range of about 0.90 to 1.40.As noted above, however, as the C/S Ratio of slag increases, itsviscosity may also increase. Because of the competitive market for ironand steel in the United States of America, U.S. iron smelters strive tooperate their blast furnaces efficiently, i.e., to produce "lean" slagswith low C/S Ratios, low Base Numbers, and, consequently, lowviscosities. For example, at Bethlehem Steel's Sparrows Point Facilityin Sparrows Point, Md., U.S.A., slag C/S Ratios are generally in therange of about 0.95 to 1.05. Where furnace operating conditions and rawmaterials differ, slags may be produced with higher C/S Ratios. Slagswith C/S Ratios greater than about 1.25 or with Base Numbers greaterthan about 1.55, however, are not suitable for this process.

It is preferred that the chemical content of the molten slag isdetermined before the source of CaO is mixed with it. This may be doneby standard sampling techniques and laboratory analysis. Blast furnaceoperators commonly run historical chemical analysis on each casting, sothat the efficiency of the blast furnace operations can be continuouslymonitored. Nevertheless, for this process, sampling equipment,laboratory testing equipment, and trained laboratory personnel arepreferably available for real time testing of the molten slag'scomposition. Automated testing equipment capable of continuously testingand reporting the chemical composition is preferred for accomplishingthis step. While various chemical tests can be performed, it is alsopreferred that at least the CaO, MgO, SiO₂, and Al₂ O₃ contents aremeasured, and the C/S Ratio and Base Number are calculated.

It is also preferred that the rate of flow of the molten slag ismeasured. The rate of flow of the molten slag can be measured in tonsper minute (TPM). For example, at Bethlehem Steel's Sparrows PointFacility, the rate of flow of the slag may average about three (3) TPMper slag runner, but may range between about zero (0) and ten (10) TPM.The rate of flow of the slag may be adequately calculated by observingthe depth of the slag in the slag runner and determining the rate of theflow of molten slag past a point on the slag runner. Preferably,however, the rate of flow of the molten slag may be measured by anautomatic flowmeter which can provide continuous measurement of the rateof flow of the molten slag.

If the chemical composition and rate of flow of the molten slag areknown or can be calculated by the means described above, the amount ofthe source of CaO required to achieve a desired range of C/S Ratios andBase Numbers also can be calculated. The preferred source of CaO isquicklime, and dolomitic quicklime is the preferred type of quicklime.Quicklime is preferred because it fuses with the molten slag attemperatures in the range of about 2600° to 2650° F. (1226° to 1250°C.). Therefore, no additional heat or fuel to produce heat may need tobe added to the molten slag to perform the process. When using quicklimeas the source of CaO, current tests have shown that increases in the CaOcontent of the molten slag of up to about 4.0 percent +/-0.5 percent maybe attained without the addition of fuel to generate additional heat inthe molten slag.

If lime, such as slaked or hydrated lime, is the source of CaO, it maybe necessary to add a low volatile fuel, e.g., coke fines or rice hulls,to the molten slag in order to generate sufficient heat to de-hydratethe lime. It also may be necessary to add oxygen (O₂) to the molten slagto facilitate the combustion of the low volatile fuel.

If, however, limestone is the source of CaO, it also may be necessary toadd a low volatile fuel to the molten slag in order to generatesufficient heat to decarbonate the limestone, i.e., reduce the CaCO₃ toCaO and CO₂. It also may be necessary to add oxygen (O₂) to the moltenslag to facilitate the combustion of the low volatile fuel.

The mixing of the source of CaO and the molten slag can be accomplishedin several ways. The least expensive and, therefore, a preferred methodis to supply a source of CaO ground to various degrees of fineness, suchthat the source of CaO will sink into the molten slag and dispersethroughout the molten slag as it dissolves. It is preferred that ifquicklime is the source of CaO, the quicklime should be crushed orground to an average diameter of about 0.375 inches (0.953 cm). It isalso preferred that the source of CaO does not contain an excessiveamount of fines. Fines tend to float on the surface of the molten slagand to saturate the surface layer of the molten slag with CaO, as theydissolve. This local saturation may cause the C/S Ratio and theviscosity of the surface layer of the molten slag to increase rapidly.The increase in the C/S Ratio also may cause the crystalline phasepreference of the surface layer to increase. A layer of crystalline slagmay then form on the surface of the molten slag and prevent the desiredmixing of the molten slag below the crystalline layer and the source ofCaO.

There are several methods to inhibit the formation of such a crystallinelayer and to encourage proper mixing, including the physical mixing ofthe molten slag and the source of CaO and the addition of means forcreating turbulence in the molten slag at the mixing point. A preferredmethod is to add a volatile fuel, such as a wooden board, to the moltenslag at the mixing point. This volatile fuel will burn in the moltenslag and produce gasses that will percolate through the slag and mayaccelerate the mixing of the molten slag and the source of CaO. Othermeans for creating turbulence in the slag flow at the mixing point aredisclosed in the description of the system for performing this process.

After the source of CaO has been mixed with the molten slag, the moltenslag is granulated at a granulating station. Granulating the molten slagcomprises breaking up and quenching the molten slag by means of aquenching fluid. The purpose of granulating the molten slag is to obtainvitreous granulated slag, e.g., granulated slag with a glass content ofat least about 90 percent. If the quenching fluid(s) is(are) injectedinto the slag at sufficiently high pressure, it(they) may break up themolten slag into small pieces. Small pieces may cool more rapidly thanlarger pieces and may achieve a higher degree of vitrification. Slag,which is permitted to cool slowly and, therefore, to crystalize, mayexhibit little or no cementitious qualities. Further, as the C/S Ratioof molten slag increases, so does its melting temperature therebyincreasing the risk of crystalline phases forming at a given slagtemperature.

Preferably, the quenching fluid is a liquid or a gas, or both, whichis(are) injected into the molten slag at a temperature sufficiently lowto quench the slag and in sufficient quantity and under sufficientpressure to granulate the molten slag. It is further preferred that thequenching fluid is water at ambient temperature and that it is injectedinto the molten slag at about ten (10) times the rate of flow of themolten slag, e.g., if the rate of flow of the slag is about three (3)TPM, water is preferably injected into the slag at a rate of about 30TPM, and at a pressure of about 30 psi (1551 torr).

Regardless of the quality of the granulation, it may not be possible toachieve a degree of vitrification greater than in the range of about 90to 95 percent. Nevertheless, some slag crystal formation may not only beunavoidable, but indicative of the efficiency of the mixing of CaO andthe molten slag and of the granulation. After granulation, microscopy orx-ray diffraction (XRD) scans can be used to determine the degree ofvitrification of the granulated slag. At C/S Ratios greater than about1.06, merwinitic crystallites have been detected in granulated slag.Merwinite (Ca₃ MgSi₂ O₈) crystallizes at temperatures in the range ofabout 2600° to 2650° F. (1226° to 1250° C.) at high C/S Ratios. Thepresence of some merwinitic crystals may indicate that the molten slagwith a C/S Ratio in the range of about 1.06 to 1.25 was properlygranulated. When slag is cooled too slowly, melilitic crystallites havebeen detected by microscopy and XRD scans. Melilite ((Na,Ca)₂(Mg,Al)(Si,Al)₂ O₇) crystallizes at a temperature of about 2400° F.(1132° C.). The molten slag normally must cool at a relatively slow ratethrough this temperature in order for melilitic crystallites to form.Therefore, it is preferred that some merwinitic crystallites aredetected to confirm the efficiency of the granulation. Further, thepresence of these merwinitic crystallites has shown few or no factorsdisadvantageous to the slag's hydraulic qualities or cementitiousproperties of cementitious materials manufactured from the slag.

The granulated slag may then be ground to a predetermined degree offineness. The degree of fineness may depend on the desired hydrationrate of the cementitious material and the chemical composition of thegranulated slag. As noted above, the hydration rate of such cementitiousmaterials is dependent in part on the degree of fineness of thegrinding. Because cementitious materials produced with increased C/SRatios in accordance with this process may exhibit increased hydraulicreactivity, a coarser grind of these cementitious material may be usedto obtain the properties of a finer grind of other granulated slags.This results in higher production rates of cementitious materialsproduced by this process.

A regression analysis was performed comparing the degree of fineness ofthe grind measured in Blaine for various samples and the percentagechange in CaO content of the slag and the Seven (7) Day SAI values. Overthe course of this analysis, one (1) micron was found to equal about 615Blaine. The Seven (7) Day SAI values were used because they are thecritical values in the ASTM C989, incorporated herein by reference,testing. The following relation was found to exist between thevariables:

    7 Day SAI=0.01 (Blaine)+3.91 (CaO)-119.94 (+/-3.15 percent at 99 percent confidence).

Solving for the derivatives of this equation, i.e., (d 7 Day SAI)/(dCaO) and (d 7 Day SAI)/(d Blaine), yielded the following results:

    One (1) percent Change in CaO=3.91 percent change on 7 Day SAI 391 Blaine=3.91 percent change on 7 Day SAI.

Therefore, a change of about one (1) percent in CaO content should beequivalent to a change of about 391 Blaine or about 0.64 microns offineness.

Referring to FIG. 2, the graph depicts the relationship betweenincreases in median particle size, due to increases in slag CaO content,and increases in ball mill grinder production rates. Ball mill grinderproduction rates are measured in tons per hour (TPH). Median particlesize is indicated in microns as measured by a Microtrac. Point Arepresents current ball mill grinder production at a predetermineddegree of fineness. Addition of a source of CaO sufficient to raise theCaO content of the granulated slag by about one (1) percent permits theslag to be less finely ground and increases the ball mill grinderproduction rate. The increase in both variables is identified by Point Bin FIG. 2. A two (2) percent increase in CaO content would permit thegranulated slag to be even less finely ground and would increase theball mill grinder production rate by 8.7 TPH. See FIG. 2, Point C.

In order to convert the ground granulated slag into a cementitiousmaterial with properties similar to those of Portland Cement, anactivator must be added. Preferably, the activator is an alkali selectedfrom a group consisting of potassium hydroxide, sodium hydroxide, andcalcium hydroxide. It is further preferred that the activator isobtained by combining the ground, granulated slag with Portland Cement.When water is added to Portland Cement, an excess of calcium hydroxideis produced which creates the preferred alkaline conditions foractivating the ground, granulated slag.

The following example is a further illustrative disclosure of theprocess described above.

EXAMPLE

Samples of granulated slag were taken at Bethlehem Steel's SparrowsPoint Facility (1) during normal blast furnace operations and (2) afterdolomitic quicklime had been added to the molten slag in accordance withthe process described above. These samples were dried in an oven atabout 318° F. (150° C.), so that the water content of the granulatedslag is reduced to a range of about two (2) to three (3) percent. Thesamples were then analyzed in a laboratory, and their chemicalcomposition, glass content (or degree of vitrification) as determined byXRD scan in accordance with British Standard BS6699, and crystallinephase as determined by powder XRD scan are set forth in Table II. Thistable indicates that the quicklime added to the molten slag reacted withand was assimilated into the slag. As a result, the C/S Ratio rose from0.99 in the normal slag to 1.07 in the improved slag. And the BaseNumber increased from 1.06 to 1.14 without adversely affecting the glasscontent of 97 percent.

                  TABLE II                                                        ______________________________________                                                                   Slag with                                                          Normal     addition of                                                        Slag       Quicklime                                          Sample          1          2                                                  ______________________________________                                        Chemical Analysis %                                                           SiO.sub.2       37.5       36.3                                               (IR)            (0.14)     (0.18)                                             Al.sub.2 O.sub.3                                                                              9.8        9.4                                                Fe.sub.2 O.sub.3                                                                              1.0        0.7                                                CaO             37.1       38.8                                               LOI (N.sub.2)   0.1        <0.1                                               LOI (Air)       +(0.8)     +(0.8)                                             MgO             12.9       13.1                                               Mn.sub.2 O.sub.3                                                                              0.65       0.41                                               P.sub.2 O.sub.5 0.01       0.01                                               TiO.sub.2       0.44       0.35                                               SO.sub.3 + S as SO.sub.3                                                                      (1.73)     (1.97)                                             S               0.66       0.76                                               K.sub.2 O       0.23       0.24                                               Na.sub.2 O      0.25       0.29                                               Free Quicklime  <0.1       <0.1                                               CaO/SiO.sub.2   0.99       1.07                                               Base Number     1.06       1.14                                               (CaO + MgO)/SiO.sub.2                                                         Hydraulic Index 1.59       1.69                                               =    CaO + MgO + Al.sub.2 O.sub.3                                                 SiO.sub.2                                                                 % Glass content 97         97                                                 (BS6699)                                                                      Crystalline phases                                                                            Trace of   Merwinite and a                                    identified by XRD                                                                             Merwinite  trace of Calcite                                   ______________________________________                                    

Portions of each of these samples were ground in a laboratory ball millgrinder to a Blaine fineness, as defined by ASTM C204, of about 5000 and6000 cm² /g. Each of these ground portions was then mixed with anOrdinary Portland Cement (OPC) to form a 50--50 blend. These mixtureswere tested along with unblended OPC to determine their seven (7) andtwenty-eight (28) day mortar strengths in accordance with ASTM C109 andtheir two (2), seven (7), and twenty-eight (28) day mortar strengths inaccordance with EN196-1. The results of these tests are presented inTable III. A standard laboratory ball mill grinder was also used toestimate the energy required to grind each sample to a Blaine finenessof about 4500, 5000, and 6000 cm² /g. These estimates are presented inTable IV.

                                      TABLE III                                   __________________________________________________________________________                           Normal Slag     Slag with addition of Quicklime                        Plymstock                                                                            (No. 1)         (No. 2)                                Samples         OPC    50/50 OPC/Slag Bends                                                                          50/50 OPC/Slag Blends                  __________________________________________________________________________    Specific Surface Area, cm.sup.2 /g                                                            4080   4500 5000  6000 4500 5000  6000                        ATSM-C204                                                                     ATSM-C109 Compressive                                                         Strengths PSI                                                                 7 day           4334   4125 4371  4600 4179 4665  5294                        % of OPC        100    95.2 100.9 106.1                                                                              96.4 107.6 122.2                       % of normal slag blend                                                                        --     100  100   100  101.3                                                                              106.7 115.1                       (Equivalent fineness)                                                         28 day          5402   5860 6171  6394 6101 6101  6999                        % of OPC        100    108.5                                                                              114.2 118.4                                                                              112.9                                                                              112.9 129.6                       % of normal slag blend                                                                        --     100  100   100  104.1                                                                              104.1 109.5                       (Equivalent fineness)                                                         EN 196 Compressive Strengths                                                  N/mm.sup.2                                                                    2 day           31.6   11.7 12.4  13.2 12.8 13.4  14.1                        % of OPC        100    37.0 39.2  41.8 40.5 42.4  44.6                        % of normal slag blend                                                                        --     100  100   100  109.4                                                                              108.1 106.8                       (Equivalent fineness)                                                         7 day           46.4   27.4 29.1  32.0 32.9 35.5  40.4                        % of OPC        100    59.1 62.7  69.0 70.9 76.5  87.1                        % of normal slag blend                                                                        --     100  100   100  120.1                                                                              122.0 126.3                       (Equivalent fineness)                                                         28 day          56.6   56.7 58.9  59.7 59.6 63.8  65.0                        % of OPC        100    96.8 100.5 101.9                                                                              101.7                                                                              108.9 110.9                       % of normal slag blend                                                                        --     100  100   100  105.1                                                                              108.3 108.9                       (Equivalent fineness)                                                         __________________________________________________________________________

These estimates may be higher than the energy required to grind suchslags on a commercial scale. In commercial grinding, a grinding aid,such as a liquid chemical agent, e.g., triethanolamine or monopropyleneglycol, may be used in conjunction with a ball mill grinder to improvethe efficiency of the ball mill grinder. Further, the ball mill grinderair may be swept to remove fines, and the ground, granulated slag may becontinuously passed through a separator, so that fines and sufficientlyground slag will not interfere with the grinding process. Nevertheless,these results suggest the difference between the relative grindabilityof the slag, i.e., the ease or difficulty of grinding the slag samples,and the relative amounts of energy necessary to grind the samples to apredetermined degree of fineness.

                  TABLE IV                                                        ______________________________________                                                              Slag with addition of                                            Normal Slag  Quicklime                                                        (No. 1)      (No. 2)                                                            Gross     SSA      Gross   SSA                                     Samples    kWh/tonne cm.sup.2 g.sup.-1                                                                      kWh/tonne                                                                             cm.sup.2 g.sup.-1                       ______________________________________                                                   1.27      7.0      1.7     10                                                 29.47     1750     31.72                                                      33.44     2080     33.54   1970                                               37.67     2190     36.54   2090                                               39.90     2280     41.74   2210                                               44.37     2450     49.15   2430                                               51.07     2680     59.52   2710                                               63.67     3130             3080                                               81.43     3720     72.83   3540                                               101.44    4190     95.04   4090                                               134.71    4920     128.32  4920                                               167.94    5520     161.55  5520                                               196.77    5960     194.78  6040                                    Estimated Energy                                                              Requirements                                                                  (Gross                                                                        kWh/tonne)                                                                    At Specific                                                                   Surface Area                                                                  5000 cm.sup.2 g.sup.-1                                                                   140            133                                                 5500 cm.sup.2 g.sup.-1                                                                   168            162                                                 6000 cm.sup.2 g.sup.-1                                                                   198            192                                                 ______________________________________                                    

Based on the information provided in Tables III and IV, it can beconcluded that over a fineness range of about 5000 to 6000 cm² /g, theslag to which quicklime was added requires about four (4) percent lessenergy to grind to a predetermined degree of fineness than normal slag.Further, by mixing quicklime with the molten slag, the hydraulicreactivity of the slag increased such that in order to achieve the sameseven (7) day strengths, normal slag must be ground at least about 1000cm² /g more finely than the improved slag. The process, therefore, wouldresult in a savings of about 33 percent of the energy required forgrinding. Moreover, to produce ground, granulated slags with the sameseven (7) day strength as normal slag with a surface area of 5000 cm²/g, the surface area of the improved slag could be lowered by at leastabout 300 cm² /g. This would result in an energy savings of about 16percent over the energy required for grinding normal slag.

FIG. 3 represents a preferred embodiment of a system for performing theprocess disclosed above. The dimensions and capacities indicated aredictated in part by the blast furnace operations conducted at BethlehemSteel's Sparrows Point Facility. Referring to FIG. 3, means fordelivering a source of CaO 10 preferably comprises a railcar 11 and aspur (not shown) and piping 13 for offloading the source of CaO fromrailcar 11 to a centrally located storage bin 20. Alternatively, adirect fill pipeline 14 may be used to pipe a source of CaO from a ship,barge, or other facility to storage bin 20. Preferably, direct fillpipeline 14 has a diameter of at least about 5 inches (12.7 cm). A dryair compressor 12 provides dry compressed air to piping 13 forpneumatically transferring the source of CaO from railcar 11 to storagebin 20.

Storage bin 20 comprises a bulk storage chamber 21, a pneumatic transfervessel 24, and piping 23 for placing storage chamber 21 in communicationwith transfer vessel 24. Storage chamber 21 is pressurized with drycompressed air, so that the source of CaO can be transferred fromstorage chamber 21 to transfer vessel 24. It is preferred that storagechamber 21 has a capacity of about 200 to 300 short tons. A dustcollector 22 is mounted on storage chamber 21 to collect the causticdust released during the delivery of the source of CaO to storagechamber 21.

Means of transferring the source of CaO 30 from transfer vessel 24 toinjection bin 40 comprises transfer piping 31 which joins transfervessel 24 to injection chamber 41 and at least one, but preferably two,dry air compressors 32 supply dry compressed air to pneumaticallytransfer the source of CaO from transfer vessel 24 to injection chamber41. It is further preferred that at least one dry air compressor 32produces dry compressed air at pressures sufficient to transfer thesource of CaO at a rate of about six (6) TPM.

Injection bin 40 comprises injection chamber 41, a volumetric feeder 44,and piping 43 for connecting injection chamber 41 to volumetric feeder44. Preferably, injection chamber 41 has a capacity of about 30 shorttons. It is preferred that the source of CaO is transferred frominjection chamber 41 to volumetric feeder 44 by means of dry compressedair. Alternatively, however, injection chamber 41 can be mounted abovevolumetric feeder 44, and the source of CaO can be gravity fed tovolumetric feeder 44. It is further preferred that volumetric feeder 44is capable of delivering a source of CaO at delivery rates between aboutthree (3) and seven (7) TPM. A variety of suitable volumetric feeders,such as the Acrison 102Z, are commercially available. Preferably,suitable volumetric feeders should be capable of feeding the source ofCaO at variable rates up to about 300 cubic feet (8.496 cubic meters)per hour. Alternatively, however, the source of CaO may be mixed withthe molten slag by a feeder that measures the weight, rather than thevolume, of the CaO to be added. A dust collector 42 is mounted oninjection chamber 41 to collect caustic dust released during thetransfer of the source of CaO to injection chamber 41.

The source of CaO is mixed with molten slag 52 in a slag runner 51 at amixing point 50. Slag runner 51 is preferably constructed of fire brickor castable refractory material. Preferably, mixing is accomplishedsupplying appropriately ground quicklime to slag 52 by means ofvolumetric feeder 44. Slag runner 51 may also be equipped with a meansfor creating turbulence in slag 52 at point of mixing 50. A preferredmeans for creating turbulence is a waterfall-type step (not shown)formed in slag runner 51. It may not always be possible to add such astep to an existing slag runner because the gradient of the slag runnerwas established when the blast furnace was built. In these cases, it maybe preferred that a baffle projection (not shown) is mounted on the sideof slag runner 51 or a ripple generating projection (not shown) ismounted on the floor of slag runner 51. These wedge-shaped projectionsmay cause slag 52 to fold back on itself and create the desiredturbulence at mixing point 50.

Slag runner 51 may be equipped with means for sampling (not shown) slag52 as it flows from the blast furnace. These means for sampling are acomponent of the means for determining the molten slag's chemicalcomposition (not shown). Slag runner 51 may also be equipped with meansfor measuring the rate of flow (not shown) of slag 52 in slag runner 51.Preferably, the means for determining the chemical composition of themolten slag are capable of the continuous sampling of slag 52 and thecontinuous analysis of such slag samples. It is also preferred that themeans for measuring the rate of flow of slag 52 is a flowmeter (notshown) capable of continuously measuring the rate of flow of slag 52 inslag runner 51. It is further preferred that the output from the meansfor determining the chemical composition of slag 52 and the output fromthe means for measuring the rate of flow of the slag 52 are provided toa volumetric feeder control unit (not shown) such that volumetric feeder44 can be automatically adjusted to provide sufficient amounts of thesource of CaO to increase the C/S Ratios of slag 52 to the range ofabout 1.06 to 1.25.

Preferably, means for collecting and removing caustic dust releasedduring mixing of the source of CaO and slag 52 are located at mixingpoint 50. Slag runner 51 is preferably enclosed in a dedusting chamber53. A fan (not shown) draws air and caustic dust from dedusting chamber53 and into a scrubber 60. It is preferred that scrubber 60 is equippedwith means for separating the air and the caustic dust 61, such as waterspray heads. It is further preferred that scrubber 60 is equipped withmeans for collecting the caustic dust 62 and a return line 63 forreturning the collected caustic dust to slag 52. Alternatively, a drydust collector (not shown) could be used to remove caustic dust from theair. In a dry dust collector, air and dust are passed through apermeable fabric on which the dust collects. Dust is then recovered fromthe fabric by use of compressed air or by mechanical means, e.g.,shaking. Use of dry dust collectors, however, is limited by to the heatto which the fabric can be subjected.

It is preferred that slag runner 51 directs slag 52 to a granulatingstation (not shown). The granulating station is preferably equipped withat least one fluid jet outlet (not shown) for injecting fluid(s) at atemperature sufficiently low to quench the molten slag and in sufficientquantity and under sufficient pressure into the molten slag. This fluidinjection quenches and breaks up the slag, thereby granulating it.Preferably, the fluid is water at ambient temperature, and it isinjected into the molten slag at a rate of about ten (10) times the rateof flow of the molten slag and at a pressure of about 30 psi (1551torr).

Although wet granulated slag can be ground, it is preferred that thegranulated slag is dewatered as part of the step of grinding thegranulated slag. This can be accomplished by heating the granulated slagto promote evaporation of the excess water. Dewatering by this method,however, may require the expenditure of additional energy and thecreation of additional structures in which to heat the wet, granulatedslag. The preferred method of dewatering the granulated slag is simplyto transfer the granulated slag to a filter bed area in which the excesswater can drain from the granulated slag naturally. Alternatively, thegranulated slag may be dewatered by use of a rotating slag drum whichcan operate at a relatively low speed and uses gravitational force todewater the slag.

It is further preferred that as part of the step of grinding thegranulated slag, the water content of the granulated slag is reduced toa range of about two (2) to three (3) percent. If necessary, a dryer orintegral grinder drying chamber may be a preferred means of reducing thewater content to the desired range.

Before the granulated slag is ground, its chemical composition should bedetermined and the degree of fineness required to obtain a predeterminedreactivity should be calculated. Means for measuring the granulatedslag's chemical composition (not shown) preferably comprise a means fordrawing a sample of the slag after granulation and means for analyzingthe sample to determine its chemical composition. It is furtherpreferred that the measuring of the chemical composition is continuousand that the output of this measuring is provided to a grinder controlunit (not shown). Preferably, the grinder control unit will instruct thegrinder (not shown) as to the degree of fineness desired, given thedesired characteristics of the cementitious material and the chemicalcomposition of the granulated slag.

It is also preferred that the grinder (not shown) is a ball mill (ortube) grinder. Such grinders are generic commutation grinders and areavailable from a variety of commercial sources. A ball mill grindercomprises a large diameter, steel cylinder which is filled with castalloy balls of sizes that may range between about 0.625 and 4 inches(1.588 and 10.16 cm) in diameter. The granulated slag enters one end ofthe cylinder and is ground between the balls as the cylinder is rolled.The ground, granulated slag discharged from the other end of thecylinder is separated into two categories. Slag that has beensufficiently ground can be added together with an activator to form acementitious material. Slag that is too coarsely ground is returned tothe grinder for further grinding. Alternatively, a high pressure rollcompression grinder may be used instead of or in conjunction with theball mill grinder.

The preferred means of adding an activator to the ground granulated slagis to mix it with Portland Cement and add water. The reaction of thePortland Cement and the water produces excess quantities of calciumhydroxide. The mixing of the ground granulated slag with the appropriateamount of Portland Cement is preferably performed by commerciallyavailable proportioners.

The dimensions and capacities provided for the various components of thesystem may be varied to accommodate different blast furnaces,facilities, and production parameters. Although a detailed descriptionof the present invention has been provided above, it is understood thatthe scope of the process and the system are not to be limited thereby,but are to be determined by the claims which follow.

I claim:
 1. A system for producing a cementitious material from molten ferrous slag comprising:a volumetric feeder for delivering measured amounts of a source of CaO at a rate to molten ferrous slag, said molten slag containing CaO, MgO, and SiO₂, in a slag runner and a baffle projection affixed to said slag runner's side wall for creating turbulence in said molten slag, whereby said a source of CaO is mixed with said molten slag at a mixing point, so that a ratio of CaO to SiO₂ in a range of about 1.06 to 1.25 is obtained, and said slag has a Base Number less than about 1.55, wherein said Base Number equals (CaO+MgO)/SiO₂ ; a granulating station to produce granulated slag at which water at ambient temperature is are injected into said molten slag at a rate about ten times said molten lag's rate of flow and at a pressure of a least about 30 psi (1551 torr) to granulate said molten slag; and a ball mill for grinding said granulated slag to a degree of fineness to obtain a hydration rate.
 2. The system of claim 1 further comprising means for collecting and removing caustic dust released during mixing of said source of CaO with said molten slag.
 3. The system of claim 2 wherein said means for collecting and removing caustic dust comprises a dedusting chamber surrounding said mixing point; a fan, whereby air and said dust are drawn from said dedusting chamber; and a scrubber, whereby said air and said dust are separated.
 4. The system of claim 1 wherein said source of CaO is dolomitic quicklime.
 5. The system of claim 1 wherein said source of CaO is lime and further comprising means for mixing a low volatile fuel to said molten slag.
 6. The system of claim 4 further comprising means for injecting oxygen into said molten slag.
 7. The system of claim 1 wherein said source of CaO is limestone and further comprising means for mixing a low volatile fuel to said molten slag.
 8. The system of claim 7 further comprising means for injecting oxygen into said molten slag. 